Drawing the Curtain Back on Pluto and Charon

I have been interested in space and astronomy, particularly the solar system, for as long as I can remember; at least since I was in preschool. And of all the planets and other bodies in the solar system, the one that I found most fascinating in childhood was Pluto. Of course I am hardly alone in having Pluto as my “favorite planet”, but as to why I especially liked Pluto, it is a little difficult to say. I think it was in part simply that it was different; it was clearly unlike either the so-called terrestrial planets close to the Sun or the gas giants in the outer solar system. It’s status as the most distant planet, with an incredibly long orbit of 248 Earth years, may have played a role. Or maybe it was just that Pluto was so mysterious. When I first became interested in astronomy, most books had practically no information about Pluto at all, as next to nothing was known. Some of the astronomy books I would check out of the library, books that were already somewhat out of date at the time, even included some of the more wild early theories about Pluto, such as the suggestion that Pluto was in fact a fairly large planet, but was almost perfectly black, so that we were seeing the reflection of the sun off its surface like a mirror. This theory was developed to accommodate the idea that Pluto was indeed the theorized Planet X that was supposedly affecting the orbits of Uranus and Neptune (which would require a fairly massive planet) and yet failed to show a disc in even the largest telescopes of the day. More commonly, books of the time would declare Pluto to have a diameter of about 5000 km (3600 miles), which was in truth more of an upper limit than an estimate based on any concrete evidence, or repeat the then-common theory that Pluto was an escaped satellite of Neptune (this idea even crops up occasionally today in articles by non-scientists despite being long discredited; the truth is Pluto never comes anywhere remotely close to Neptune due to an orbital resonance, and actually gets closer to Uranus than it ever does to Neptune). In truth, even though it had been almost 5 decades since Pluto had been discovered by Clyde Tombaugh, the only things known with any degree of certainty about Pluto in those days was its orbital characteristics and that it had a rotation period of 6.39 days.

The mystery around Pluto began to clear up ever so slightly in the late 1970s. First methane was detected on its surface. Since methane ice is highly reflective, this was the first hint that Pluto was in fact even smaller than had been previously thought (with the constant downward revisions in estimates of Pluto's mass from the time of its discovery, at some point an astronomer joked that if the trend continued, it would soon disappear entirely!). Then in 1978 came the dramatic discovery of Pluto’s satellite - or perhaps companion would be a better term - Charon, which at half the diameter of Pluto is the largest moon with respect to its parent planet in the solar system (how hard it is to get a decent image of Pluto from Earth is illustrated by the fact that in the discovery photos of Charon, it appears as a fuzzy lump on the bigger fuzzy lump of Pluto - it took the Hubble Space Telescope to get a picture of the two clearly separated). The discovery of Charon in turn allowed Pluto’s mass to be calculated with some precision, showing that it was actually quite small for a planet, being considerable less massive than even the Earth’s Moon, let alone Mercury, the smallest inner planet, though still much larger than Ceres, the biggest asteroid. This discovery gave rise to the first arguments among astronomers about whether Pluto should be classified as a planet. But for me, while Pluto was now a little less mysterious, it remained as fascinating as ever, especially with the relative size of Charon making it more like a double planet system, if a small one (see here for an example of how Pluto and Charon orbit each other).

I don’t want to delve into the debate over Pluto’s planetary status in great detail here, but there are a few things worth pointing out. First of all, as I mentioned above, while Pluto certainly is very small in comparison with the eight universally recognized major planets, it is still much larger than any asteroid, even Ceres (by far the biggest asteroid). At least one other recently discovered body in the outer solar system, Eris, is somewhat more massive than Pluto (though slightly smaller in diameter), but that alone doesn’t disqualify Pluto from planetary status, it just means that Eris itself also needs to have its status determined, and that it should be classified with Pluto. It should also be noted that the eccentricity of Pluto’s orbit is also irrelevant to whether or not it is a planet, as quite a few extrasolar planets have similarly eccentric orbits. I am not so attached to Pluto’s planetary status that I don’t recognize that it, along with Eris and several other large trans-Neptunian objects, are distinct from the major planets; in fact way back at the end of the 1980s I distinctly remember doing a high school project in which I suggested that Pluto belonged to a completely new class of objects (even though, with the possible exception of Chiron, an asteroid-like body orbiting between Saturn and Uranus, no other similar objects had been discovered at that time) which I think I called cometoids or something like that. But essentially I think both sides are arguing about the wrong question. The argument shouldn’t be about Pluto per se; it should be about the minimum characteristics for a planet, wherever it is located. The definition that the IAU came up with when it reclassified Pluto, Eris and several other objects (including Ceres) as “dwarf planets” is vague and unsatisfactory. While any definition is going to be somewhat arbitrary, I think it makes more sense to define a planet based on its own physical characteristics, instead of vague criteria like “clearing its orbit”, as even in the outer reaches of our own solar system we may someday find an object the size of Mercury or Mars orbiting among smaller objects in the way Pluto does, and extrasolar planetary system will no doubt have all kinds of mixes of objects, whereas I think a definition of a planet should such that the same object would always be a planet if it is orbiting a star, regardless of what else may be orbiting in its neighborhood. What the qualifying characteristics should be is of course debatable, though several good possibilities, such as the object being large enough to maintain hydrostatic equilibrium, would result in Pluto and similarly sized objects being classified as planets. The point is that the definition should not be written either to deliberately include or exclude Pluto, but should be based as much as possible on clearly defined physical characteristics. Probably the easiest solution for the time being would be to keep the term “dwarf planet”, but to acknowledge that, as the name implies, a dwarf planet is still a planet, while we can refer to the eight bigger planets as “major planets” (though in that case we'd still need a definition for "major planets"). But in the end what we call Pluto is not that important, as it remains equally interesting regardless (for more arguments on both sides, see here and here).

In subsequent decades we learned even more about Pluto, first through mutual occultation events between Pluto and Charon, and then in the past few years through observations by the Hubble Space Telescope, including the discovery of four small moons of Pluto, Nix, Hydra, Kerberos and Styx. But despite these advances, our knowledge of Pluto and its companions remained extremely limited, and the best images we had of it were extremely fuzzy, due to its great distance and small size. All of that has finally changed with the flyby of the Pluto system by the New Horizons spacecraft on July 14. As a longtime “Plutophile”, it goes without saying that this is an event I have long awaited. But rather than attempting to summarize New Horizons’ discoveries so far myself (after all, one of the most exciting and dramatic things about it is the pictures, which are best seen elsewhere), I am providing some links to articles about the flyby and some of the discoveries that have been made so far, with many more to come as data continues to be sent back by New Horizons over the coming months.

Articles on Pluto's moons from the months prior to the close encounter can be found here and here.

An article on the significance of the New Horizons mission can be found here, and one about how great a value it is for the money is here.

Articles reporting on the flyby can be found here, here, here and here, with articles focusing on Pluto's atmosphere here and here, articles on the strange mountain on Charon here and here, and an article about the first relatively close-up picture on Nix here (one with pictures of both Nix and Hydra is here). A picture gallery of photos from the New Horizons mission, starting from the most recent images, can be found here.

An article on reactions to the flyby can be found here, and an article on possible future exploration by New Horizons can be found here. Unfortunately, there are no near future plans to send probes to Eris, now the largest object in the solar system that has never been visited by a space probe, or Sedna, which has by far the greatest average distance from the Sun of any known object of substantial size (it probably qualifies as a dwarf planet), but with this flyby of Pluto we have at least begun the exploration of the bodies of the far outer solar system, and after decades of wondering what they were like, I personally am happy to finally get to see what Pluto and Charon look like up close.